This invention relates to electric lithium cells, preferably used as lithium secondary cells having an extended cycle life.
Lithium secondary cells have drawn a great attention as rechargeable high energy density cells. Lithium secondary cells have the problem that when metallic lithium is used as the negative electrode active material, dendrites grow on the negative electrode as a result of charging and discharging operations. It was recently proposed to use a lithium alloy, particularly a lithium-aluminum alloy as the negative electrode.
The lithium-aluminum alloys known in the art are generally classified into the following two groups.
(1) A metallurgical lithium-aluminum alloy which is obtained by melting a mixture of lithium and aluminum in a proper ratio in an inert atmosphere.
(2) An electrochemical lithium-aluminum alloy which is obtained by electrodepositing lithium on aluminum in an organic solvent containing a lithium salt.
The metallurgical lithium-aluminum alloy (1) is preferably obtained in the form of a sheet when it is used as a cell electrode. However, metallurgical lithium-aluminum alloys currently available in the form of a sheet are alloys of a composition having an extremely low or high content of lithium. Since these lithium-aluminum alloys have a composition approximate to aluminum or lithium alone, they do not exhibit the properties characteristic of lithium-aluminum alloys. It is thus difficult to use these alloys in practice as the cell electrode.
The electrochemical lithium-aluminum alloys (2) are available as alloys having a ratio of lithium to aluminum of approximately 1:1. They are obtained generally in the form of a sheet and initially perform well as the cell electrode. However, we have found that electrochemical lithium-aluminum alloys have several problems when they are actually used as the cell electrode, namely, negative electrode. As charging and discharging operations are repeated, the alloy electrode undergoes cracking or failure to deteriorate electrical contact. Some cells can be charged or discharged no longer. Other cells can be charged, but cannot be discharged for unknown reason. The electrochemical lithium-aluminum alloys have not been acceptable in practice.
Although the prior art lithium-aluminum alloys are superior to lithium alone, they still suffer from serious problems when actually applied as a secondary cell electrode, particularly with respect to cycle properties.